1. Field of the Invention
The present invention relates to an electrode assembly and a secondary battery having the same, and more particularly, to an electrode assembly having high stability and reliability and a secondary battery having the same.
2. Description of the Related Art
In general, secondary batteries, unlike primary batteries, are rechargeable and have been used intensively as main power of, for example, communication apparatuses, information processing apparatuses, and audio/video portable apparatuses. In recent years, the secondary batteries have drawn attention and have been rapidly developed because they have a very light weight, a high energy density, a high output voltage, a low discharge rate, and a long life span and are eco-friendly.
The secondary batteries are classified into nickel-metal hydride (Ni-MH) batteries and lithium ion (Li-ion) batteries according to the type of electrode active materials. In particular, the lithium ion batteries may be divided into a lithium ion battery using a liquid electrolyte and a lithium ion battery using a solid polymer electrolyte or a gel-type electrolyte according to the type of electrolyte. In addition, the batteries are classified into a can-type battery and a pouch-type battery according to the shape of a container for accommodating an electrode assembly.
Since the lithium ion battery has a considerably higher energy density per weight than that of the primary battery, the lithium ion battery can be manufactured as a super-light weight battery. In addition, in the lithium ion battery, the average voltage of each cell is 3.6 V, which is three times higher than the average voltage 1.2 V of other secondary batteries, such as a nickel-cadmium battery and a nickel-metal hydride battery. And, the lithium ion battery has a discharge rate of less than approximately 5% per month at a temperature of 20° C., which is one third of the discharge rate of a nickel-cadmium battery and a nickel-metal hydride battery. In addition, since the lithium ion battery does not use a heavy metal, such as cadmium (Cd) or mercury (Hg), it is an eco-friendly battery, and the lithium ion battery can be charged and discharged 1000 times in a normal state. For 7 these reasons, in recent years, the lithium ion battery has been rapidly developed with the development of information and communication technologies.
In a secondary battery according to the related art, an electrode assembly including a positive electrode plate, a negative electrode plate, and a separator is housed in a can made from aluminum or aluminum alloy, a cap assembly is provided on an upper part of the can to close up an upper opening of the can, an electrolyte is injected into the can through an electrolyte injection hole, and the electrolyte injection hole is closed up, thereby forming a bare cell. The positive electrode plate is coated with a positive electrode coating portion made from positive active materials, and the negative electrode plate is coated with a negative electrode coating portion made from negative active materials. When the can is made from aluminum or aluminum alloy, the weight of the battery can be reduced since aluminum is a light material, and there is an advantage that the battery is not corroded even when it is used in a high-voltage environment for a long time.
A sealed unit bare cell is connected to safety devices, such as a PTC (Positive Temperature Coefficient) device, a thermal fuse, and a Protective Circuit Module (PCM), and other battery accessories in a separate hard pack, or it is molded with a hot-melt resin. In this way, the outer appearance of the battery is formed.
Meanwhile, the separator of the electrode assembly is provided between the positive electrode plate and the negative electrode plate to prevent a short circuit between the positive electrode plate and the negative electrode plate. The separator also serves as a safety device for preventing the overheating of the battery.
When the battery is overcharged or frequently charged and discharged, however, a silver white lithium dendrite may be deposited on the surface of negative electrode active materials, and the lithium dendrite may pass through the separator such that the positive electrode active materials contact the negative electrode active materials, which results in a short circuit between the positive and negative electrode active materials.
In addition, when a nail passes through the battery during a nail penetration test, which is a safety test for testing an internal short circuit, the active materials may be detached from collectors, and the positive electrode active materials and the negative electrode active materials detached from the collectors may contact each other, resulting in a short circuit.
Further, when the electrodes are wound together, the adhesion between the collectors and the electrode coating portions being lowered due to repeated expansion and shrinkage of the electrode plates caused by charge and discharge, the electrode coating portions may be cracked and are likely to be detached from the collectors. When the positive and negative electrode active materials are detached from the corresponding collectors, a short circuit occurs between the positive electrode active materials and the negative electrode active materials due to the contact between the positive electrode active materials and the negative electrode active materials, which lowers the stability and reliability of the battery.
Therefore, when the adhesion between electrode coating portions and the electrode collectors is lowered, it is necessary to change the structure of the electrode assembly to prevent the detachment of the electrode coating portions.